“They noticed his black jeep and noticed he appeared to be staring into the bus and looking at them.”

Mr Coxon said Ketteridge had only one hand on the steering wheel, adding: “It was clear to them what he was doing.”

One of the girls told police she looked away in disgust. “I was very scared,” she said.

The court heard Ketteridge was driving erratically, slowing so the minibus had to overtake.

One of the teachers jotted down his number plate and he was later caught.

But just three days after appearing in court for the first time, Ketteridge committed a similar crime in front of three girls aged 13 to 16 on a flyover above the A1 in Cambridgeshire. Twenty minutes later he exposed himself again to the same girls at a service station.

He committed both crimes while at work, as his job – not disclosed in court – involves him driving regularly.

Jailing him for nine months, and banning him from the roads for 12 months, Judge Peter Kelson QC said Ketteridge had committed ‘two very serious examples of offences of exposure’.

“Driving on a motorway while doing what you were doing cannot be regarded as anything other than extremely unsafe behaviour,” he said.

The court heard he has a previous conviction for indecent exposure.

Ketteridge, from Mansfield, initially denied flashing, claiming he was in urinating in a bottle due to diabetes.